BRASILIA, May 19 (Xinhua) -- Brazil's former Federal Supreme Court President Joaquim Barbosa called for protests on Friday to demand the "immediate" ouster of President Michel Temer.

Ongoing corruption investigations have reached the nation's highest office, with mounting allegations of wrongdoing nipping at Temer's heels, though he has denied the charges.

"There is no other way out. Brazilians have to mobilize, take to the streets and strongly call for President Michel Temer's immediate resignation," Barbosa posted on his website.

Barbosa, who became the first Brazilian of African descent to head the Federal Supreme Court, is a highly respected public figure.

Political and social activists, as well as union leaders, are also calling for nationwide demonstrations to keep up the pressure on Temer to step down.

Brazilian media have revealed last month that Temer had solicited 12 million U.S. dollars from Brazilian construction giant Odebrecht for his political campaigns when he was vice president.

That news "alone ... would be enough of a reason to unleash a call for his resignation," said Barbosa, adding business and political interests, along with media outlets, "have minimized the seriousness of the events."

The latest charges against Temer, which came out on Wednesday, involve a recorded conversation between Temer and meatpacking tycoon Joesley Batista, in which they openly discussed bribery.

"Now terrifying revelations from Mr. Joesley Batista have been made public about the same person, Temer," said Barbosa.

On Thursday, Temer refused to step down from office, adding that the Supreme Court's investigation would prove his innocence.

Global ratings agency Fitch announced on Friday it would maintain Brazil's rating at "BB," two notches below investment grade with a negative outlook.

"Recent political events related to President Temer have increased uncertainty regarding the reform process and could hurt broader confidence and economic recovery prospects," the agency said.

Meanwhile, a top executive at Brazilian meatpacking giant JBS told prosecutors on Friday his company paid Temer millions in bribes to protect the company's interests.

As part of a plea bargain deal, JBS Public Relations Director Ricardo Saud said holding company J&F had paid Temer nearly 15 million reals (4.6 million U.S. dollars) since 2014.

JBS and other meatpackers have been the target of a recent federal investigation into adulterated and substandard meat that undermined the country's meat exports, following revelations that the companies have paid government inspectors to overlook the irregularities.

According to Saud, Temer "acted on several occasions in favor of the interests of the companies belonging to the (J&F) group."

Prosecutor General Rodrigo Janot said Temer and Senator Aecio Neves worked together to prevent an extensive corruption investigation, known as Operation Car Wash, from making any headway, by silencing potential witnesses.

"We are also seeing the possible obstruction of justice," said Janot.